Production of cyanamide reaction products



Patented Mar. 13, 1945 y 2,371,480 v PRODUCTION OF CYANAMIDE REACTION l raonoc'rs Robert C. Swain, Riverside, and Joseph H. Paden,

I 'Glenbrook, Oonn., assiznors to American y anamid Company, New York, N. Y.,- a. corpora- 'tion of Maine 1 No Drawing. Application April 29, 1943,

' 1 Serial No.485, 074 M 3 Claims; (01. zoo-249.5)

This invention relates to the conversion of cyanamide and its dimer, dicyandiamide, to other organic nitrogen compounds by processes involving the heating of these materials athigh temperatures and pressures in the presence of solvents containing ammonia. More particularly, the invention relates to the production of products from these materials substantially free from contamination by iron by carrying out the conversion in autoclaves consisting of or lined with iron alloys of'the typehereinafter described. cyanamide and dicyandiamide have recently become importantraw materials for the production of a number of organic nitrogen compounds. 'I'hus, for example, melamine can be produced by heating a charge consisting of 1 part of cyanamide ordicyandiamide, from 1-2 parts'of anhydrous ammonia and 1-2 parts-of methyl alcohol or other organic solvent at temperatures of 120-200 C. for 4-12 hours. This process can also be modified as described in Patent No. 2,221,478, dated November 12,1940, to produce 'guanidine salts by introducing equivalent quantities of ammoniumsalts'into the autoclave. I

It is a principal object of the present invention to improve these and similar conversionprocejsses involving the reaction of cyanamide or di'cyandiamide in the presence of liquid ammonia by con- 7 ducing them in the presence only of certain alloys of iron and chromium, or of iron-chromium a1- loys containing certain relatively small quantities of nickel of the type hereinafter defined.

It is a well-known fact that ordinary ammonia,

both aqueous and anhydrous, is not corrosive to,

ordinary iron or steel, even at elevated temperatures. Thus, for example, we have heated samples of hot rolled mild steel boiler plate for 50 hours at temperatures of 180 C. while suspended g in a mixture of equal quantities of methanol and anhydrous ammonia without any measurable attack on the metal. It is also a known fact that neither cyanamide nor dicyandiamide willattack j iron or steel'at ordinary temperaturesandjin fact, these compounds have been shipped in ordi- 'nary iron'containers for many years. Solutions of cyanamide and of dicyandiamide'in liquid ammonia, with or without dilution with methylorethyl alcohol do not attack iron or steel 'at orjdi- "nary temperatures. It is therefore very surpris-,.

ing to find, 'as we have found, that when such solutions of cyanamide or dicyandiamide in liquid ammonia, with 01' without the addition of alcohols or other organic solvents are heated to conversion 'temperaturesof 120-200 C. in an ordinary steel also encountered when guanidine salts are produc'ed;v Non-ferrousequipment'capable of withstanding such pressures safely, and of a size sulficient to produce the necessary commercial' outputs, would be extremely expensive tirclesigi'rand maintain.

A We have nowdiscovered a classoi ferrousalloys that are not attacked during'the conversion of cyanamide-ammonia and dicya'ndiam1de-.

ammonia solutions, either as such or containing ammonium salts for the production of 'guanidine salts, under the conditions hereinafter described. "These alloy are known commerciallyaas highchro me steels, since they are alloys of iron with about -8-20%"of chromium based" on the total weightof the 'alloy. The'alloys are referred .to as Io w'carbon" steels, since they usually'contain from about 0.04% to-0.2% of carbon, and'they may'also contain other minormaterials such as small amounts'of sulfur, selenium, manganese, and silicon. The alloys of this class preferably contain'nickel to improve their machining properties and ease of fabrication, but wehave found I that substantial corrosion occurs if an'ironchromium alloy containing morethan 15%"of nickel is used for the purposes of 'the invention.

- We have'also found, as an important feature of taniumfl These" stabilizers are preferably" em- 'ployed in amounts equivalent to ten times the amount of carbon present-in the alloy or more.

We are not able to state thereasonfor the extremely corrosive action that is'attendant" upon the conversionofcyanamide or dicyandiamide to melamine,- g'uanidine salts or other conversion products in the presence vofliquid ammonia at elevated temperatures of -200 C. Several, unusual corrosionphenoinena appear in carrying'out this reaction; thus; for example, ordinary autoclave the autoclave is rapidly attacked and'55' unstabilized' 18-8- iron-chromium-nickelj alloy 'is corroded at a relatively rapid rate, whereas a I similar alloy containing small amounts of molybdenum, columbium or titanium is not corroded.

' On the other hand, a straight iron-chromium almuch as 150 parts per million of dissolved iron,

as would be the case if a corrodible steel or iron alloy were present in the liquid in the autoclave.

Dissolved iron in the reaction mixture apparently acts as a catalyst or accelerator of corrosion, and for this reason it is highly important to construct the inner surfaces of the autoclave of metal that I v is not initially corroded. We find thatthis initial corrosion can be prevented by the incorporation of small quantities on the order of 25-4000 parts v per million, based on the totalweight' oi fthei charge, of an inhibitor such as phosphoric acid (H3PO4), ammonium phosphate, or a-soluble idle chromate such as sodium, potassium or ammonium dichromate, which operates to counteract the corrosive action of any iron that may be dis solved from the autoclave or introduced with the raw materials, and this constitutes another important feature of our invention.

I Our invention therefore consists in the conversion of cyanamide or dicyandiamide to a reaction product thereof by heating the compound while dispersed in a solvent containing liquid ammonia to reaction temperatures of 120-200" C. in a closed pressure vessel in which substantially all the internal surfaces in contact with the reaction mixture consist of a high chrome steel of one of the types defined above. This high chrome steel may be an alloy consisting of 8-20% or chromium and 0.04 to;0.2 of carbon, the balance of the alloy being substantially all iron with small amounts of impurities of the type outlined above. Prefer- Kably, however, the alloy consists of 8-20% of chromium, about -8-12% of nickel (with a maximum of about 0.04 to 0.2% of carbon and about 0.45% or slightly more of a stabilizerof the group consisting of molybdenum, columbium and titanium, or mixtures thereof, the balance being iron, and minor materials such as sulfur or selenium andmanganese, which improve the machining properties of the alloy when used in amountsbf less than about 2%. Any of the finish and prepared for testing. Two samples of high-chrome steels containing no nickel were also prepared in a similar manner. The samples had the analysis indicated in Table I.

Table I Per cent composition Per cent stabilizer Bal- Sample No.- 32$: G Cr. Ni Si Mn Mo Cb Ti 0.15 '12 7 0.12 18 Autoclaveliner l8 8 0.50

The samples were weighed and were then strung on a glass tube and separated by'means of small glass tube spacers. They were then suspended in an autoclave by means of fibre glass cloth so pended it was charged with a mixture containing equal parts by weight of dicyandiamide, liquid ammonia. and commercial methanol and was then closed and heated in the usual manner. -The autoclave required about one hour t reach a temperature of 180 C. and it was then held at above described 01" similar conversions of cyan amide or dicyandiamide in solvents containing ammonia may be carried out at elevated temperatures and pressures in autoclaves consisting of. or

lined with the above described alloys without excessive corrosion when care; is taken to avoid initial or pro gresssive contamination oithe charge with dissolved iron.

The invention will be illustrated in greater detail by thefollowing specific examples, which show the extent of corrosion that occurs withvarious alloys in practicing the processes of the invention. .It should be understood, however, that various modifications in the compositions of the alloysmay be resortedrto within the scope of the appended claims, and that the invention initsbroader aspects is not limited to the specific examples'giveni Cosaosron Tnsrs' 0' Five disk samples of iron-chromium' nickel a1- loys, some of which. were stabilized with molyb- -190 C. for 2 hours, after which it was cooled "quicklyto room temperature, the melamine discharged and another charge was put in. The

samples were subjected to a total of 23 such runs 1 which amounted to a total of 46 hours at 180-190 C. in addition to the heating and cooling periods.

At the end of this time the samples were removed from the autoclave, cleaned with boiling water, dried and weighed. They were then eX- amined microscopically for surface etchin Pits and cracks. The data on corrosion, weights, weight losses, etc., of these samples are given in Table II.

Table II Wt. (grams) Wt. (grams) Loss in Area.- cfmosipn sample before test after test grams (11111. 1

Reference to Table II above will show thatfmost.

.of the corrosion occurred in sample No. 5, which contalned20% of nickel, and sample No. 3, which was an ordinary unstabilized "18-8 alloy. This was confirmed by the appearance of the samples under the microscope as they were the most badly stained and etched samples of the lot.

Microscopic examination of the remaining almost complete lack of corrosion. Samplesfi and 7, containing no nickel, also showed an almost complete absence of change under the experimental conditions, when judged by the appearases cir'the disksbefoi'eand after e posuret the i j st i"'e'action"which confirms the low -corrosi'on 'ratein'di'cated by the data cf Table i. v

EXAMIPLEZ an autoclave was fabricated from a high chrome steel having the following specifications:

. Per cent Chromium 17.5-20 Nickel 8-10 Carbon 1 0.2 Molybdenum 1 0.6 Sulfur or selenium 1 0.07

Guanidlne nitrate was prepared in this autoclave by the following method? 168 grams of di-'' cyandiamide containing 5 parts per' million or iron and 320 grams of ammonium nitrate con": taining 12 p. p. m. of iron were dissolved in 300 grams of liquid ammonia and heated to 160 C.

The autoclave was heated at; 25 160 C. for 1 hour longer and was then cooled and discharged. The product was pure white in about 2 hours.

guanidine nitrate of excellent color having the following analysis:

Percent Guanidine nitrate; l 94.1 Dicyandiamide 0.0 NH-iNOS 0.18

The metallic impurities were determined spectroscopically, and were as follows:

On the basis or theresults shown by Example Per cent Iron .0020 Chromium .0003 Nickel .00005 Melamine was also prepared in this autoclave from dicyandiamide having the same iron analysis, using the procedure outlined in Example 1. The product was found to contain 0.0008% of iron, 0.0003% of nickel and 0.00005% of chromium.

An autoclave of about 10 gallons capacity was constructed with an inner liner and internal heating coils and agitator composed of an alloy having the following composition:

Per cent Chromium g 18.37

- Nickel 10.34

Manganese 0.44

Silicon 0.42 Carbon 7 0.05

Columbium (stabilizer) 0.77

Iron Balance duced a product whichwas brown in color and contained considerable quantities of iron, chromium and nickel. Thus, for example, the products from the 'first two runs contained thefollowing quantitiesfof metal in parts per million:

moval of this nutthe remaining batches were. light in color and the content of metallic impurities was almost negligible, averaging 27 parts per million of ironand less than 3 parts per million of Cr and Ni, combined, for a total of 12 additional runs. The high content of chromium and nickel in the first batches indicated that considerable corrosion was taking place in the alloy liner of the autoclave, while the very small values obtained after removal of the steel nut (which contained no chromium or nickel) showed that this corrosion was the direct result of the presence of substantial amounts of dissolved iron during the reaction.

This application is a continuation-in-part of our copending application Serial No. 348,556, filed July 30, 1940.

What we claim is:

1. In the conversion of a compound selected from the group consisting of cyanamide and dicyandiamide to a reaction productthereof, the steps which comprise repeatedly heating dispersions of said compound in a solvent comprising liquid ammonia at temperatures of 1 20-200 C. in a closed pressure vessel the internal surfaces of which consist of a high chrome steel selected from the group consisting of .those having the composition:

Chromium 820% Carbon 0.04-0.2% Iron Balance of alloy and Chromium 13-20% Carbon 0.04-0.2% Nickel Up t0'15% Stabilizer 0.45% Iron Balance of alloy said stabilizer being a member of the group consisting of molybdenum, columbium, and titanium,

said process being carried out with substantially iron-free reagents in such a manner and under such conditions that dissolved iron is only slowly built up in the reaction mixture and stopping the reaction before the iron content reaches 0.015% of the reaction mixture, whereby corrosion of the metal in contact with the-reaction is minimized.

2.In the conversion of a compound selected from the group consisting of cyanamide and dicyandiamide to a reaction product'thereof, the steps which comprise-repeatedly heating dispersions of said compound in a solvent comprising liquid ammonia at temperatures of -200". C

in a closed pressure vessel the internal surfaces of which consists of high chrome steel having the following composition:

Balance of alloy said stabilizer being a member of the group condiscovered that the manufacturer 4 can-4&0

dating 0! molybdenum, columbium, and titani- 3. A method according to claim 1 inwhtcb a. um, said process being carried out with suhstansmall amount of an;inhibitor seiected-from the tially iron-free reagents in such manner and group consisting of soluble phosphates and d1- under'such conditions thatdissolved iron in the chromates is present in the reaction mixture. reaction mixture is only slowly built up and stop- 6 ping the reaction before the iron content reaches ROBERT C. SWAIN. 0.015% of the reaction mixture, whereby cor- 9 JOSEPH rosion of the metal in contact with the reaction 1 wisminimized. 

